This invention relates to sorting or identification of materials, and more specifically, to non-destructive analysis of an object or container to identify its make-up or contents, based on its dielectric behavior under an oscillating electric field.
All packaged materials can be characterized by an effective electrical permittivity which can be measured as a dielectric constant, and by an effective a.c. conductance. This is true whether the materials inside the package are homogeneous or non-homogeneous. Several techniques and apparatus for nondestructive testing and classification of the objects' materials have been proposed wherein the object is disposed between a pair of parallel capacitor plates, an a.c. voltage is applied to the plates, and the effective capacitance across the plates is measured. Because different materials have different electrical properties, it is theoretically possible to classify the materials into one of two or more categories, e.g., lean or fatty meats, dry or moist soils, explosive or inert baggage contents. Several prior proposals along these lines are described, e.g., in U.S. Pat. Nos. 4,263,511; 4,370,611; 4,433,286; and 4,493,039.
The prior techniques, while showing some promise, tended to overlook factors that prevented optimal operation. For example, the positions of the parallel plates were fixed, so that the interplate spacing could not be adjusted to match closely the thickness of the object. Also, there was no provision for measuring the capacitance across only the effective cross section of the test object. Accordingly, it was impossible to compensate either for bending of the field lines due to the presence of the dielectric (or conductor) between the plates, or for divergence of the electric field lines at edges of the capacitor plates. Also, none of these systems included a bridge circuit that was resistant to inherent d.c. drift. None of them took into account the fact that stray capacitance between the plate and the bridge circuit could introduce unacceptable errors into the regained measurements.
Accordingly, numerous problems with the methods and apparatus so far proposed, i.e., those arising from the non-ideal characteristics of the sensors, have imposed serious problems. These problems include unused air space in between the sensor plates, bending and bulging of the field lines near edges of the sensors and within the objects under test, and non-ideal characteristics of the materials that are used to hold and orient the test objects between the sensors. In addition there is normal, statistical variation of electrical properties from one test object to the next, and there are changes in these properties caused only by the passage of time. Some of the electrical effects of the test object can be masked by quantities of undesired materials in the test object that are buried in more massive amounts of the normal material within the test object. These and other sample-related anomalies continue, with sensor related problems, to further limit or negate the usefulness of the apparatus and technique.